Line cards for key telephone circuits are a highly developed art in which one of the earliest patents using the present approach is U.S. Pat. No. 3,436,488 issued Apr. 1, 1969 to R. Barbato et al. A number of patents followed such as U.S. Pat. No. 3,647,983 issued Mar. 7, 1972 to A. Fitzsimmons et al, U.S. Pat. No. 3,715,516 issued Feb. 6, 1973 to J. Ebrahimi and U.S. Pat. No. 4,064,373 issued Dec. 20, 1977 to E. Pinede et al. The first named of these is the well-known Bell System 400-D line card which is and has had heavy usage in the key system area. The second noted and last noted patents show commercially sold variations known as the San/Bar 4000 card and the ITT 400-E card.
Given that line cards have been in existence for many years, it has now been mandated by the Federal Communications Commission that protection be imposed between the key system and the nationwide telephone network. Protection of a number of types is acceptable.
This protection may be inserted in the wiring of the premises and may be fully protected, protected against contact with commercial power wiring, or may be designed for protection without the use of protective devices.